Detection of ultrasound using optical techniques has gained increasing interest during recent years. Resonant optical structures, such as etalons, fiber gratings, and dielectric multilayer interference filters, can be employed as ultrasound sensing elements. These devices rely on the interaction of an optical field confined in a resonance cavity with a transient ultrasonic field. The interaction takes place due to the modulation of the optical properties of the resonance cavity in response to the strain induced by the ultrasonic field. The sharp resonance line of the cavity amplifies the optical response to the transient strain. The advantage of using optical methods over conventional piezoelectric based sensors is apparent when small element size ultrasound arrays are considered. The effective area of an optically based ultrasound sensor is determined by the size of the confined optical field, which in most cases can be scaled down to micrometer size without increasing detection noise. For the case of a piezoelectric sensing element, however, signal noise increases with reduced element size.
Microresonator detectors can rely on accurate measurement of the effective refractive index change of the guided mode inside waveguides, due to the presence of biomolecules on the surface of sensing areas or in the solution surrounding the devices. In microresonator detectors, signals can be detected by measuring resonance shifts, or alternatively, by measuring output intensity changes from the device at a fixed wavelength. The latter technique is especially useful for detecting very small changes in the effective index. Such a property can be used to detect, sensitively, the resonator's response to an incident ultrasound pulse. Accordingly, this arrangement can be used for high-frequency ultrasound detection. Moreover, ultrasound imaging with high-spatial resolution can be achieved using arrays of integrated microring resonators.
Very high frequency ultrasound imaging at the frequency range of 30-100 MHz is capable of resolving structures almost down to the cellular level. Developing such an imaging modality for clinical use could have a tremendous impact on the diagnostic and therapeutic procedures in many different clinical areas. The cardio-vascular clinician will be able to visualize in great detail the arterial walls of the coronary arteries and the heart interior structure. The diagnostics of cancer using biopsy will be revolutionized as in-situ microscopy could replace the traditional procedure. Imaging guided therapy could be developed since the diagnosed pathology can be localized at a great precision.
Currently intravascular ultrasound (IVUS) imaging is used in arterial wall imaging for cardiovascular diagnostics. The resolution attained by these devices exceeds 100 μm (more than 200 μm laterally). Due to their limited resolution current IVUS devices are unable to show early stages of atherosclerosis, or to identify thin fibrous caps, a hallmark of plaques believed to be most susceptible to rupture.
According to the principles of the present teachings, a method is provided that is based on optical microresonators of very high quality factor acting as highly sensitive ultrasound receivers. These microresonators, designed using integrated optics techniques, are formed using closed-loop (ring type) shaped waveguides. A typical dimension of such a microresonator is 20 μm to 60 μm depending on the optical wavelength and other design parameters. Preliminary measurements showed an extraordinary high sensitivity giving rise to a high signal to noise ratio of about 30 using a driving acoustic signal of 60 KPa power. These experiments were performed using a microring resonator of moderate quality factor (Q=1000) excited using relatively low optical power of 1.5 mW. These results imply that a microring array having an element spacing of less than 20 μm could deliver high sensitivity and provide image resolution that is at least five times better that of any existing IVUS system. Such a system can utilize separate optical elements acting as ultrasound generators whose principle relying on photoacoustic ultrasound generation.
The design of a 1-D or 2-D microring array using integrated optics device techniques according to the present teachings offers unique advantages such as ruggedness, small size, RF immunity, and low manufacturing cost, which could be beneficial in various ultrasound applications. A particularly appealing medical application is the design of a small integrated optical device which will operate as an intravascular imaging probe. The ultrasound pulse generator could be integrated using photoacoustic methods, therefore eliminating the need for any electrical cabling since fiber optics carry both input and output signals. The high element density required for high resolution intravascular imaging dictates an upper limit on the ring diameter. Reducing the size of the rings will also increase the free spectral range and therefore will increase the number of elements that can share a common bus waveguide.
Further areas of applicability of the present teachings will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present teachings.